In U.S. Pat. Nos. 6,869,206, 6,960,872, 7,025,464, 7,040,774, and 7,048,385, there is shown an enhanced light source, which can be formed by placing LEDs in a light recycling cavity. These light recycling cavities can enhance the brightness of the LEDs which are used to form them, thereby achieving gains in brightness greater than 2
Forming these light recycling cavities requires precision placement of the LEDs and reflecting surfaces inside the cavity and also a means to dissipate the heat generated by the LEDs to exterior heatsinks. For example, in U.S. Pending Patent Application Publication Number US-2006-0092639-A1, “High Brightness Light Emitting Diode Light Source”, to William R. Livesay et al., these LED light recycling cavities can be integrated with heatsinks.
However, not shown is a simple, inexpensive, and mass producible means of forming these light recycling cavities. In these light recycling cavities, unlike most conventional mounting methods, the LEDs are not placed in a planar arrangement. The individual LEDs or an array of LEDs are aligned orthogonal to the other LEDs or LED arrays inside the cavity.
This orthogonal alignment is quite different than most electronic component fabrication processes, which utilize planar fabrication methods and planar substrates or circuit boards. A substrate and its planar arrangement are in 2 dimensions. The light recycling cavity and its orthogonal alignment is in 3 dimensions. This orthogonal alignment makes it difficult or impossible to use high speed and mass production processes like surface mount, chip on board, etc. as conventionally practiced with 2 dimensional substrates to form these light recycling cavities in 3 dimensions. For example, a light recycling cavity may be formed by mounting LEDs on a thermally conductive substrate (aluminum nitride) and forming an array of LEDs thereon utilizing prior art techniques. Multiple arrays (similarly formed) are then individually mounted onto heatsinks, which are then brought into alignment to form a light recycling cavity.
However, precise alignment of these arrays to each other and within the light recycling cavity is required. This fabrication process can be tedious and difficult, requiring that each array have intimate thermal connection to an exterior heatsink. This requires attaching each substrate to a separate heatsink and then joining all of the arrays and heatsinks together to form a cavity while maintaining alignment and precise positioning of the LED arrays to each other within the cavity. Due to the three dimensional nature of the finished cavity, it is difficult to use high speed manufacturing means such as pick and place machines to mass produce these assemblies.
Therefore, there is a need for an improved method of forming these light recycling cavities that simplifies the assembly of the light recycling cavity and maintains alignment of the LEDs within the light recycling cavity. In U.S. Pat. No. 5,997,708, Craig discloses a foldable substrate utilizing a multilayer assembly. However, this assembly is complex and requires bonding other materials to the substrate to provide a suitable hinge to effect the fold. This requires extra process and alignment steps.
In the prior art, electronic packages that have incorporated foldable elements have utilized flex circuits formed by a metal (for the electrical interconnect) deposited onto a pliable substrate usually plastic (e.g. polyimide, Mylar, etc.) to form the foldable material or hinge.
However, there are several drawbacks to these prior art techniques particularly in an assembly requiring high alignment accuracy and high thermal conductivity. Most polymers have very low thermal conductivity and low maximum service temperatures. If the polymer is used only for the hinge material, it must be somehow attached to the target substrate. This attachment requires an adhesive which must also have a maximum working temperature sufficiently high to allow die attachment processes to be performed. This die attachment process also adds an additional process step to form the foldable assembly that is not easily performed in a planar process.
The hinge, if so attached, requires that it have excellent adhesion to the substrate (to the edge of the substrate at the folding joint) so that alignment of the LEDs of the light recycling cavity is maintained when the substrate is folded. Most polymer materials require a minimum thickness for mechanical robustness. However, the minimum bend radius is at least the thickness of the material. For example, if a 0.025 mm (0.001″) thick polyimide layer is used as the hinge material, this requires at least at least a 0.025 to 0.050 mm or greater bend radius. This variable bend radius imparts an uncertainty to the position of the substrate parts when folded. A requirement for a light recycling cavity is to have very few or small gaps between the LEDs within the cavity. This uncertainty in the position of the LEDs requires larger spacing (between LED arrays so they will not mechanically interfere) and lowers cavity efficiency.
Therefore, there is a need for a highly accurate means of forming these light recycling cavities without increasing process steps.